Method for manufacturing silicon carbide semiconductor device and the silicon carbide semiconductor device

ABSTRACT

Silicon carbide semiconductor device includes trench, in which connecting trench section is connected to straight trench section. Straight trench section includes first straight trench and second straight trench extending in parallel to each other. Connecting trench section includes first connecting trench perpendicular to straight trench section, second connecting trench that connects first straight trench and first connecting trench to each other, and third connecting trench that connects second straight trench and first connecting trench to each other. Second connecting trench extends at 30 degrees of angle with the extension of first straight trench. Third connecting trench extends at 30 degrees of angle with the extension of second straight trench. A manufacturing method according to the invention for manufacturing a silicon carbide semiconductor device facilitates preventing defects from being causes in a silicon carbide semiconductor device during the manufacture thereof.

This is a divisional of 12/628,819, filed Dec. 1, 2009, for whichbenefit is claimed, and status is pending, which in turn claims priorityfrom JP Application No. 2008-306870, filed Dec. 1, 2008, and JPApplication No. 2009-087895, filed Mar. 31, 2009, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a method for manufacturing a siliconcarbide semiconductor device and the silicon carbide semiconductordevice. Specifically, the invention relates to a method formanufacturing a silicon carbide semiconductor device having a trenchstructure and the silicon carbide semiconductor device having a trenchstructure.

B. Description of the Related Art

In the semiconductor devices such as a metal oxide semiconductor fieldeffect transistor (hereinafter referred to as a “MOSFET”) and aninsulated gate bipolar transistor (hereinafter referred to as an “IGBT”)having a trench structure, straight trenches, for example, are formed ina stripe pattern from the surface of a semiconductor substrate.

In the semiconductor devices having a straight trench structure,electric field localization occurs at the end portion of the trench,when a high voltage is applied to the semiconductor devices. When apointing trench end portion is caused by dry etching, electric fieldlocalization is liable to be caused at the pointing trench end portion.As the electric field localizes to the trench end portion and exceedsthe breakdown voltage of the semiconductor device to the higher side,there is a high probability that the semiconductor device will breakdown.

FIG. 43 is an electron micrograph that shows the result of a leakageanalysis conducted on a MOSFET having a conventional trench structure.The MOSFET is observed by emission microscopy (hereinafter referred toas “EMS”). In the MOSFET shown in FIG. 43, straight trench 101 isdisposed in a silicon carbide semiconductor device (hereinafter referredto sometimes as an “SiC semiconductor device”) that employs a siliconcarbide semiconductor substrate (hereinafter referred to as an “SiCsubstrate”). In end portion 102 (indicated by a double-dotted-chaincircle) of trench 101, a light emission caused by current leakage isobserved. In the portion that emits light as shown in FIG. 43, electricfield localization is liable to result and, therefore, the portion thatemits light as shown in FIG. 43 will break down with a high probability.

To avoid the problem described above, the trench end portion is roundedor the trench end portions are connected to each other to remove trenchend portions in the silicon semiconductor device (hereinafter referredto as the “Si device”) that employs a silicon (Si) semiconductorsubstrate.

A semiconductor device as described below that has a trench structure,in which the trench end corner section is rounded, is proposed in thefollowing Patent Document 1. In the semiconductor device proposed inJapanese Unexamined Patent Application Publication No. 2003-188379, thewidth of the trench in the vicinity of the end portion thereof is set tobe narrower in the planar shape than the width of the trench in thetrunk portion thereof. The trench is formed by dry-etching such that thedepth of the trench in the vicinity of the end portion thereof is set tobe shallower than the depth of the trench in the trunk portion thereofand the trench end corner section is rounded. By the structure describedabove, a singular point is prevented from causing on the gate oxide filmor the gate electrode in the trench end corner section to relax theelectric field localization to the trench end corner section or toprevent the breakdown voltage in the trench end corner section fromdecreasing.

The semiconductor device proposed in Japanese Patent Publication No.4130356 includes a semiconductor layer on a semiconductor substrate; afirst cell region and a second cell region in the semiconductor layer,the first cell region and the second cell region being adjacent to eachother; each of the first and second cell regions including a pluralityof stripe-shaped trench lines extending in perpendicular to the boundarybetween the first and second cell regions; each of the trench lineshaving a first end portion and a second end portion; first connectiontrenches connecting the first end portions of some pairs of the adjacenttrench lines such that the first end portions of at least a pair of theadjacent trench lines are not connected to each other; second connectiontrenches connecting the second end portions of some pairs of theadjacent trench lines such that all the trench lines in each of thefirst and second cell regions are connected to each other but such thatthe second end portions of at least another pair of the adjacent trenchlines are not connected to each other; gate insulator films in thetrench lines, the first connection trenches and the second connectiontrenches; gate electrodes buried in the trench lines, the firstconnection trenches and the second connection trenches with therespective gate insulator films interposed therebetween; a gate wiringon the semiconductor layer, the gate wiring being on the boundarybetween the first and second cell regions, the gate wiring beingconnected electrically to the gate electrodes; a first electrode on thefirst major surface of the semiconductor layer; a second electrode onthe second major surface of the semiconductor layer; and at least one ofthe first connection trenches in the second cell region not facing toany of the second connection trenches in the first cell region.

The semiconductor device proposed in Japanese Unexamined PatentApplication Publication No. 2001-168329 has a trench structure in whichadjacent trenches are connected with each other with a U-shapedconnecting trench such that the end portion of a trench facing the chipedge and the end portion of the adjacent trench are connected with eachother with the connecting trench having a large curvature and disposedin a p-type well region.

Another semiconductor device proposed in Japanese Unexamined PatentApplication Publication No. 2001-332727 has a trench structure, in whichadjacent trenches are connected with each other with a U-shapedconnecting trench such that the end portion of a trench facing to thechip edge and the end portion of the adjacent trench are connected witheach other by the connecting trench, the width thereof is larger thanthe width of the straight section of the trench.

As a result of investigation conducted by the present inventors, it hasbeen found that the following problems will be caused, if the techniquesdescribed above are applied to the SiC semiconductor device. In thefollowing, the problems caused are described, for example, in connectionwith the application of the technique proposed in Japanese UnexaminedPatent Application Publication No. 2001-168329 to the SiC semiconductordevice.

FIG. 36 is an electron micrograph of a conventional silicon carbidesemiconductor device before the heat treatment thereof from the surfaceside thereof. The SiC semiconductor device is observed under a scanningelectron microscope (hereinafter referred to as a “SEM”). (The SiCsemiconductor devices are observed also under a SEM in FIGS. 1 through31 and in FIG. 33.) Trench 111 is formed in the surface portion of theSiC semiconductor device. Trench 111 is formed of adjoining straighttrenches (hereinafter referred to collectively as “straight trenchsection”) 112 and semicircular connecting trench (hereinafter referredto as “connecting trench section”) 113 that connects the end portions ofthe adjoining trenches constituting straight trench section 112 witheach other with a semicircular curve.

In the SiC semiconductor device shown in FIG. 36, trench 111 is formedby dry-etching. In some portions 114 of connecting trench section 113,the trench width is narrowed or an uneven trench side wall occurs.(Hereinafter, these defects will be referred to as “defective trenchformations 114 caused by trench etching” or simply as “defective trenchformations 114.”) It is estimated that defective trench formations 114are caused by the variation of the dry-etching speed depending on thecrystal plane of the SiC semiconductor device (the crystal planedependence of the dry-etching speed).

FIG. 37 is an electron micrograph that shows the conventional siliconcarbide semiconductor device after the heat treatment thereof from thesurface side thereof.

The heat treatment is conducted in an argon (Ar) gas, to whichmonosilane (SiH₄) is added at the flow rate ratio of 0.4%, under thepressure of 80 Torr and at 1700° C. for 60 minutes. The SiCsemiconductor device is observed using a focused ion beam (hereinafterreferred to as an “FIB”). (The SiC semiconductor devices are observedusing an FIB in also FIGS. 38 through 42.) In the conventional SiCsemiconductor device shown in FIG. 36 and treated thermally, it isconfirmed that defective portion 115 filled occurs in connecting trenchsection 113. (Hereinafter defective portion 115 will be referred to as“defective trench formation 115.”)

The states of defective trench formation 115 are observed by cutting outthe cross sections of the SiC semiconductor device. FIG. 38 is anelectron micrograph observing the cross section along the line segmentA-A′ of FIG. 37. FIG. 39 is an electron micrograph observing the crosssection along the line segment B-B′ of FIG. 37. FIG. 40 is an electronmicrograph observing the cross section along the line segment C-C′ ofFIG. 37. FIG. 41 is an electron micrograph observing the cross sectionalong the line segment D-D′ of FIG. 37. FIG. 42 is an electronmicrograph observing the cross section along the line segment E-E′ ofFIG. 37.

A wolfram (W) protector film is deposited on the SiC semiconductordevice for protecting the surface thereof in micro-machining a specificmicro area with an FIB. FIGS. 38, 40 and 42 show the trench remaining inthe SiC semiconductor device. In contrast, the trenches in the SiCsemiconductor devices shown in FIGS. 39 and 41 are filled and almost notremaining in the SiC semiconductor devices.

In manufacturing a semiconductor device having a trench structure, aheat treatment is conducted, after forming trenches, at 1500° or higherto improve the shapes of the trenches and to activate the implantedimpurity atoms. In the SiC semiconductor device, the heat treatment asdescribed above narrows the trench in a part of the connecting trenchsection or deforms the trench to be shallow. It is found that thelocations in the connecting trench section, at which the deformationssuch as a narrowed trench and a shallow trench are caused by the heattreatments, and the magnitudes of the deformations are differentdepending on the curvature of the connecting trench section.

The curvature of the connecting trench section is determined by thespacing (cell pitch) between the trenches in the straight trenchsection. In other words, it is found that different magnitudes ofunevenness are caused at different locations on the side wall and thebottom plane of the connecting trench section depending on the cellpitch. If the portion of the connecting trench section narrowed bytrench-etching (cf. FIG. 36) is further narrowed by a heat treatment, apart of the connecting trench section will be filled. If a part of theconnecting trench section is deformed greatly by the unevenness causedby the heat treatment, the unevenness itself will cause defective trenchformation.

If the straight trench section is connected with a connecting trenchsection, a part of the connecting trench section will be filled, asdescribed above, by the heat treatment conducted at 1500° C. or higheron the SiC semiconductor device. If a part of the connecting trenchsection is filled, an end portion will be caused on the trench or thetrench end portion will be pointed. If the electric field localizes tothe trench end portion, the SiC semiconductor device will bedeteriorated or broken down.

In view of the foregoing, it would be desirable to obviate the problemsdescribed above. It would be also desirable to provide a method formanufacturing a silicon carbide semiconductor device that facilitatespreventing defects from causing during the manufacture thereof. It wouldbe further desirable to provide a non-defective silicon carbidesemiconductor device. The present invention is directed to overcoming orat least reducing the effects of one or more of the problems set forthabove.

SUMMARY OF THE INVENTION

According to the subject matter of the appended claim 1, there isprovided a method for manufacturing a silicon carbide semiconductordevice, the method forming a trench from the surface of a substrate madeof a silicon carbide semiconductor, the trench including a firststraight trench section including first straight trenches extending inparallel to each other, the method including the step of:

connecting the end portions of the adjacent first straight trenches toeach other with a second straight trench section extending at 30 degreesof angle with the first straight trench section and a third straighttrench section extending in perpendicular to the first straight trenchsection such that a polygon is formed;

connecting the end portions of the adjacent first straight trenches toeach other with a fourth straight trench section extending at 60 degreesof angle with the first straight trench section and the third straighttrench section extending in perpendicular to the first straight trenchsection such that a polygon is formed; or

connecting the end portions of the adjacent first straight trenches toeach other with the second straight trench section, the fourth straighttrench section and the third straight trench section such that a polygonis formed.

According to the subject matter of the appended claim 2, the spacingbetween the first straight trenches described in the appended claim 1and connected to each other with the second straight trench section orthe fourth straight trench section and the third straight trench sectionis from 1.7 μm to 2.5 μm.

According to the subject matter of the appended claim 3, the spacingbetween the first straight trenches described in the appended claim 1and connected to each other with the second straight trench section, thefourth straight trench section, and the third straight trench section isfrom 4 μm to 5.6 μm.

According to the subject matter of the appended claim 4, there isprovided a method for manufacturing a silicon carbide semiconductordevice, the method forming a trench from the surface of a substrate madeof a silicon carbide semiconductor, the trench including a firststraight trench section including first straight trenches extending inparallel to each other, the method including the step of:

connecting the end portions of the adjacent first straight trenches toeach other with a fifth straight trench section extending inperpendicular to the first straight trench section such that a polygonis formed by the inner side walls of the first straight trenches and theside wall of the fifth straight trench section continuous to the innerside walls of the first straight trenches.

According to the subject matter of the appended claim 5, the firststraight trenches described in the appended claim 4 are widenedgradually from the end portions thereof to the fifth straight trenchsection.

According to the invention, there is provided a method for manufacturinga silicon carbide semiconductor device, the method forming a trench fromthe surface of a substrate made of a silicon carbide semiconductor, thetrench including a first straight trench section including firststraight trenches extending in parallel to each other, the methodincluding the step of:

connecting the end portions of the adjacent first straight trenches toeach other with a sixth straight trench section extending at 20 degreesof angle with the first straight trench section, a seventh straighttrench section extending at 40 degrees of angle with the first straighttrench section, a fourth straight trench section extending at 60 degreesof angle with the first straight trench section, an eighth straighttrench section extending at 80 degrees of angle with the first straighttrench section, and a third straight trench section extending inperpendicular to the first straight trench section such that a polygonis formed.

According to the invention, the spacing between the first straighttrenches in one embodiment is from 8 μm to 12 μm.

According to the invention, there is provided a method for manufacturinga silicon carbide semiconductor device, the method forming a trench fromthe surface of a substrate made of a silicon carbide semiconductor, thetrench including a first straight trench section including firststraight trenches extending in parallel to each other, the methodincluding the step of:

connecting the end portions of the adjacent first straight trenches toeach other with a ninth straight trench section extending at 11.25degrees of angle with the first straight trench section, a tenthstraight trench section extending at 22.5 degrees of angle with thefirst straight trench section, an eleventh straight trench sectionextending at 33.75 degrees of angle with the first straight trenchsection, a twelfth straight trench section extending at 45 degrees ofangle with the first straight trench section, a thirteenth straighttrench section extending at 56.25 degrees of angle with the firststraight trench section, a fourteenth straight trench section extendingat 67.5 degrees of angle with the first straight trench section, afifteenth straight trench section extending at 78.75 degrees of anglewith the first straight trench section, and a third straight trenchsection extending in perpendicular to the first straight trench sectionsuch that a polygon is formed.

According to the invention, the spacing between the first straighttrenches in one embodiment is wider than 12 μm.

According to the invention, the side walls in the first straight trenchsection are made to coincide with the (1-100) plane and the (−1100)plane of hexagonal silicon carbide having a four-layered stackingstructure (hereinafter referred to as “4H—SiC”). In writing the Millerindices, a bar “−” is prefixed to a numeral for indicating a negativeindex.

According to the invention, the trenches are formed by dry-etching.

According to the invention, the method further includes the step ofthermally treating the trenches at 1500° C. or higher after forming thetrenches.

According to the subject matter of the invention, there is provided asilicon carbide semiconductor device including:

-   -   a silicon carbide semiconductor substrate of a first        conductivity type;    -   a trench in the surface portion of the semiconductor substrate,        the end portions of the trenches being connected to each other        such that a polygon is formed;    -   a base region of a second conductivity type disposed in adjacent        to the trench;    -   a source region of the first conductivity type disposed in the        base region; and    -   a gate electrode in the trench with a gate insulator film        interposed therebetween.

According to the invention, a trench is formed in a silicon carbidesemiconductor device such that the trench includes a first straighttrench section including first straight trenches, the end portionsthereof are connected with each other with a third straight trenchsection extending in perpendicular to the first straight trench sectionand any one or both of a second straight trench section extending at 30degrees of angle with the first straight trench section and a fourthstraight trench section extending at 60 degrees of angle with the firststraight trench section.

According to the invention, the trench sections connecting the endportions of the first straight trenches are connected to form a polygon,depending on the spacing between the first straight trenches (cellpitch), having side walls which are hardly deformed by a heat treatment(such as half the circumference of a hexagon, half the circumference ofa dodecagon, half the circumference of an octadecagon and half thecircumference of a triacontagon). Therefore, it is possible tomanufacture a silicon carbide semiconductor device that facilitatespreventing the trenches thereof from being filled by the heat treatmentconducted at 1500° C. or higher. Since it is possible to manufacture asilicon carbide semiconductor device that includes no end portions leftin the trench according to the invention, the electric field isprevented from localizing to the end portions of the trench. Therefore,it is possible to prevent the silicon carbide semiconductor deviceaccording to the invention from being deteriorated or broken down.

The manufacturing methods according to the invention facilitatepreventing defects from causing in the silicon carbide semiconductordevices.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIG. 1 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a first embodiment of theinvention.

FIG. 2 is another electron micrograph that observes the surface ofanother silicon carbide semiconductor device according to the firstembodiment of the invention.

FIG. 3 is an electron micrograph observing the surface, in which acircular trench is disposed, of a silicon carbide semiconductor devicebefore the heat treatment thereof.

FIG. 4 is an electron micrograph observing the surface of the siliconcarbide semiconductor device shown in FIG. 3 after the heat treatmentthereof.

FIG. 5 is an electron micrograph observing surface of a silicon carbidesemiconductor device according to a comparative example.

FIG. 6 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a second embodiment of theinvention.

FIG. 7 is an electron micrograph that observes the surface of thesilicon carbide semiconductor device shown in FIG. 6 before the heattreatment thereof.

FIG. 8 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a third embodiment of theinvention.

FIG. 9 is an electron micrograph that observes the surface of thesilicon carbide semiconductor device shown in FIG. 8 before the heattreatment thereof.

FIG. 10 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a fourth embodiment of theinvention, in which the cell pitch is set at 3.8 μm.

FIG. 11 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to the fourth embodiment of theinvention, in which the cell pitch is set at 6.5 μm.

FIG. 12 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to the fourth embodiment of theinvention, in which the cell pitch is set at 10 μm.

FIG. 13 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to the fourth embodiment of theinvention, in which the cell pitch is set at 15 μm.

FIG. 14 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 15 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 16 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 17 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 18 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 19 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 20 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 21 is an electron micrograph observing the surface of a trenchformed to be circular before the heat treatment thereof.

FIG. 22 is an electron micrograph observing the surface of a part of thetrench shown in FIG. 21.

FIG. 23 is an electron micrograph observing the surface of the trenchshown in FIG. 14 after the heat treatment thereof.

FIG. 24 is an electron micrograph observing the surface of the trenchshown in FIG. 15 after the heat treatment thereof.

FIG. 25 is an electron micrograph observing the surface of the trenchshown in FIG. 16 after the heat treatment thereof.

FIG. 26 is an electron micrograph observing the surface of the trenchshown in FIG. 17 after the heat treatment thereof.

FIG. 27 is an electron micrograph observing the surface of the trenchshown in FIG. 18 after the heat treatment thereof.

FIG. 28 is an electron micrograph observing the surface of the trenchshown in FIG. 19 after the heat treatment thereof.

FIG. 29 is an electron micrograph observing the surface of the trenchshown in FIG. 20 after the heat treatment thereof.

FIG. 30 is an electron micrograph observing the surface of the trenchshown in FIG. 21 after the heat treatment thereof.

FIG. 31 is an electron micrograph observing the surface of a part of thetrench shown in FIG. 30.

FIG. 32 is a table describing the curvatures of circular trenches andthe shapes of the trenches before and after the heat treatment thereof.

FIG. 33 is an electron micrograph schematically showing a siliconcarbide semiconductor device according to the invention.

FIG. 34 is a cross sectional view along the line segment X-X′ of FIG.33.

FIG. 35 is a cross sectional view along the line segment Y-Y′ of FIG.33.

FIG. 36 is an electron micrograph that observes a conventional siliconcarbide semiconductor device before the heat treatment thereof from thesurface side thereof.

FIG. 37 is an electron micrograph that observes the conventional siliconcarbide semiconductor device after the heat treatment thereof from thesurface side thereof.

FIG. 38 is an electron micrograph observing the cross section along theline segment A-A′ of FIG. 37.

FIG. 39 is an electron micrograph observing the cross section along theline segment B-B′ of FIG. 37.

FIG. 40 is an electron micrograph observing the cross section along theline segment C-C′ of FIG. 37.

FIG. 41 is an electron micrograph observing the cross section along theline segment D-D′ of FIG. 37.

FIG. 42 is an electron micrograph observing the cross section along theline segment E-E′ of FIG. 37.

FIG. 43 is an electron micrograph that shows the result of a leakanalysis conducted on a MOSFET having a conventional trench structure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Now the invention will be described in detail hereinafter with referenceto the accompanied drawings which illustrate the preferred embodimentsof the invention.

Throughout the following descriptions and the accompanied drawings, thesame reference numerals are used for designating the same constituentelements and their duplicated descriptions are omitted for the sake ofsimplicity.

In the following descriptions, silicon carbide will be hexagonal siliconcarbide having a four-layered stacking structure (hereinafter referredto as “4H—SiC”), if not specifically described otherwise.

First Embodiment

FIG. 1 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a first embodiment of theinvention.

Silicon carbide semiconductor device (hereinafter referred to as “SiCsemiconductor device”) 100 includes, for example, an SiC single crystalsubstrate or a substrate including an SiC single crystal substrate andan SiC epitaxial layer on the SiC single crystal substrate (hereinafterreferred to collectively as an “SiC substrate”) and trench 1 formed inthe surface portion of the SiC substrate. Trench 1 is formed ofadjoining straight trenches (hereinafter referred to as “straight trenchsection”) 2 and connecting trench section 5 connecting the end portionsof straight trench section 2. In the SiC substrate, a plurality oftrenches 1 is disposed almost in parallel to straight trench section 2,for example, in a stripe pattern. Straight trench section 2 correspondsto a first trench section.

Straight trench section 2 is disposed, for example, in the channelregion of SiC semiconductor device 100. First trench (hereinafterreferred to as “first straight trench”) 2 a that constitutes straighttrench section 2 and second trench (hereinafter referred to as “secondstraight trench”) 2 b that also constitutes straight trench section 2are arranged almost in parallel to each other. The side walls of firststraight trench 2 a are made to coincide, for example, with the (1-100)plane and the (−1100) plane of 4H—SiC. The side walls of second straighttrench 2 b are made to coincide also with the (1-100) plane and the(−1100) plane of 4H—SiC.

Connecting trench section 5 is disposed, for example, in a regionoutside the channel region (hereinafter referred to as an “edge region”)of SiC semiconductor device 100. Connecting trench section 5 includesstraight trench 3 extending perpendicular to straight trench section 2(hereinafter referred to as “first connecting trench 3”), straighttrench 4 a that connects the end portion of first straight trench 2 aand the first end portion of first connecting trench 3 (hereinafterreferred to as “second connecting trench 4 a”), and straight trench 4 bthat connects the end portion of second straight trench 2 b and thesecond end portion of first connecting trench 3 (hereinafter referred toas “third connecting trench 4 b”). By the formation of these connectingtrenches, connecting trench section 5 is shaped to be polygonal. Firstconnecting trench 3 corresponds to a third trench section. Second andthird connecting trenches 4 a and 4 b correspond to a second trenchsection.

Second connecting trench 4 a is disposed such that the angle betweensecond connecting trench 4 a and the extension of first straight trench2 a (hereinafter referred to as the “first connection angle”) will be 30degrees of angle, if measured from the extension of first straighttrench 2 a toward the inside of straight trench section 2. Thirdconnecting trench 4 b is disposed such that the angle between thirdconnecting trench 4 b and the extension of second straight trench 2 b(hereinafter referred to as the “second connection angle”) will be 30degrees of angle, if measured from the extension of second straighttrench 2 b toward the inside of straight trench section 2.

By setting the first and second connection angles to be 30 degrees ofangle, it is possible to prevent a part of connecting trench section 5from being filled to vanish or from being made to be shallow, forexample, by a heat treatment conducted at 1500° C. or higher. The reasonfor this will be described later.

FIG. 2 is another electron micrograph that observes the surface ofanother SiC semiconductor device according to the first embodiment ofthe invention.

As shown in FIG. 2, trenches are formed in SiC semiconductor device 110so that the first and second connection angles may be 60 degrees ofangle. The reason for this will be described later. In the structureshown in FIG. 2, second and third connecting trenches 4 a and 4 bcorrespond to a fourth trench section. The other structure in SiCsemiconductor device 110 is the same as that of SiC semiconductor device100 shown in FIG. 1.

Now the method for manufacturing the SiC semiconductor devices describedabove will be described below.

For example, an off-substrate tilted by 8 degrees of angle from the(0001) C plane of 4H—SiC or an off-substrate tilted by 8 degrees ofangle from the (0001) C plane of 4H—SiC, on which an SiC epitaxial layeris laminated, is used for the SiC substrate. At first, the SiC substrateis washed to remove the particles and organic materials on the surfaceof the SiC substrate. Then, a silicon dioxide (SiO₂) film is formed onthe (0001) C plane of the SiC substrate by plasma-enhanced chemicalvapor deposition (hereinafter referred to as “PECVD” or simply as“plasma CVD”). To form the SiO₂ film, monosilane (SiH4), oxygen (O2),and argon are used as the raw material gases. Very high frequency (VHF)electric power, for example, is fed to the CVD apparatus for theelectric power for plasma generation.

Then, the SiC substrate is washed and a photoresist is coated on theSiO₂ film using a coating apparatus and such an apparatus. Then, thepattern of the reticle, in which the trench pattern is formed, is copiedto the photoresist using a stepper. In the trench pattern, trenches,including straight trenches, the end portions of which are connected toeach other via a connecting trench section and constituting a straighttrench section, are arranged in a stripe pattern. Development isconducted to form the trench pattern in the photoresist. The trenchpattern is examined and confirmed under a microscope and then the firstbake is conducted on the photoresist. The second bake is conducted onthe photoresist in an oven to bake the photoresist hard.

Then, the SiO₂ film is dry-etched in a reactive ion etching (hereinafterreferred to as an “RIE”) apparatus using the photoresist for a mask. Forexample, a mixed gas containing trifluoromethane (CHF₃) and argon isused for a reaction gas for dry-etching the SiO₂ film. Radio frequency(RF) electric power is fed to the RIE apparatus.

The photoresist on the SiC substrate is burned to ashes with the plasmaof a reactive gas to decompose and remove the photoresist on the SiCsubstrate. For example, a mixed gas containing trifluoromethane andoxygen is used for a reaction gas. RF electric power is fed to theashing apparatus. After the ashing, the SiC substrate is immersed in aliquid for peeling off the photoresist to completely remove thephotoresist remaining on the SiC substrate. Then the SiC substrate isimmersed and washed in isopropyl alcohol (IPA). The SiC substrate iswashed with pure water and dried.

Trenches are formed by dry-etching in the surface portion of the SiCsubstrate using the SiO2 film for a mask. An etching apparatus which usean inductively coupled plasma (hereinafter referred to as an “ICP”) isused for the dry etching. Sulfur hexafluoride, oxygen, and argon areused for the reaction gases. ICP electric power for plasma generationand electric power for a bias power supply are fed to the etchingapparatus.

The SiC substrate is immersed in hydrofluoric acid (HF) for 30 minutesor longer to remove all the SiO₂ film. Then, the SiC substrate istreated thermally at 1500° C. or higher. The thermal treatment isconducted, for example, in an argon gas, to which monosilane is added atthe flow rate ratio of 0.4%, under the pressure of 80 Torr and at 1700°C. for 60 minutes. A source region and a drain region are formed on theSiC substrate by ion implantation to form, for example, a MOSFETstructure on the SiC substrate. Thus, an SiC semiconductor device iscompleted.

The second bake facilitates improving the resistance of the photoresistagainst reactive ion etching, for example. By thermally treating the SiCsubstrate after forming the trenches, the micro defects caused in thetrenches and the unevenness caused on the trench side walls are removedand the trenches are brought into a good shape.

The SiC semiconductor devices shown in FIGS. 1 and 2 are manufactured bythe method described above. An off-substrate tilted by 8 degrees ofangle from the (0001) C plane of 4H—SiC is employed. The SiO₂ film isformed by plasma CVD. The SiO₂ film is set to be 2.5 μm thick. Informing the SiO₂ film, the pressure inside the CVD chamber is set at 50Pa. Monosilane, oxygen and argon are fed to the CVD chamber for the rawmaterial gases. VHF electric power of 60 MHz and 500 W is fed for theelectric power for plasma generation. The substrate temperature duringthe film formation is set at 400° C. For the light exposure, a reticle,in which a trench pattern, 1.5 μm wide, is formed, is employed. Thefirst bake is conducted at 100° C. for 1 minute. The second bake isconducted at 120° C. for 15 minutes. The photoresist is about 2.5 μmthick after the second bake.

An RIE apparatus is used for patterning the SiO₂ film. The pressureinside the etching chamber is set at 3 Pa. A mixed gas containingtrifluoromethane and argon is introduced for the reaction gas into theetching chamber at the respective flow rates of 10 sccm and 10 sccm. RFelectric power of 75 W is fed to the etching chamber. The ashing isconducted in an ashing chamber, the inside pressure of which is set at150 Pa. A mixed gas containing trifluoromethane and argon is introducedinto the ashing chamber at the respective flow rates of 4 sccm and 100sccm. RF electric power of 150 W is fed to the ashing chamber.

The trench etching on the SiC substrate is conducted in an ICP etchingapparatus. The pressure inside the etching chamber is set at 2 Pa.Sulfur hexafluoride, oxygen and argon are fed as the reaction gases tothe etching chamber at the respective flow rates of 8.5 sccm, 1.5 sccmand 50 sccm. ICP electric power of 450 W is fed for the electric powerfor plasma generation and electric power of 8 W is fed as the bias powersupply. Trenches are set to be from 3.5 to 4.5 μm deep. Since sideetching occurs, the resulting trench width is 2 μm. The orientation flatof the SiC substrate is set on the (1-100) plane or the (−1100) plane.Trenches are formed such that the straight trench sections are parallelto the orientation flat. The trench side walls are made to coincide withthe (1-100) plane and the (−1100) plane of 4H—SiC. After the trenchesare formed, a heat treatment is conducted in an argon gas, to whichmonosilane is added at the flow rate ratio of 0.4%, under the pressureof 80 Torr and at 1700° C. for 60 minutes.

It is investigated how the trenches disposed on the SiC substrate aredeformed by the teat treatment conducted at 1700° C. FIG. 3 is anelectron micrograph observing the surface, in which a circular trench isdisposed, of a silicon carbide semiconductor device before the heattreatment thereof. FIG. 4 is an electron micrograph observing thesurface of the silicon carbide semiconductor device shown in FIG. 3after the heat treatment thereof.

As shown in FIG. 3, trench 10 having an almost circular shape(hereinafter referred to as “circular trench 10”) is formed on the(0001) C plane of SiC substrate 120. Circular trench 10 is shaped almostwith the same curve in any of the directions such as the [11-20]direction, [1-100] direction, [−1100] direction, and [−1-120] directionof 4H—SiC.

SiC substrate 120 including circular trench 10 as described above istreated thermally at 1700° C. for 60 minutes. The manufacturing methodfor manufacturing SiC semiconductor device 120 and the conditions forforming the trench are the same as those for SiC semiconductor device100 shown in FIG. 1.

It is found from FIG. 4 that circular trench 10 is deformed by the heattreatment conducted at 1700° C. The side wall of circular trench 10extending at 15 degrees of angle, at 45 degrees of angle or at 75degrees of angle with any of the [11-20] direction, [1-100] direction,[−1100] direction and [−1-120] direction of 4H—SiC is deformed andcorners are caused on the side wall of circular trench 10. In contrast,the side wall of circular trench 10 extending at 30 degrees of angle, at60 degrees of angle or at 90 degrees of angle with any of the [11-20]direction, [1-100] direction, [−1100] direction and [−1-120] directionof 4H—SiC is not deformed. In other words, it is found that circulartrench 10 is deformed by the heat treatment conducted at 1700° C. totrench 11 having an almost dodecagonal shape (hereinafter referred to as“polygonal trench 11”).

The formation of polygonal trench 11 may be attributed to thedeformations of the trench side walls, the magnitudes of which changedepending on the vigorousness of the vaporization, coagulation andsurface diffusion of silicon and carbon (C) in the heat treatment. Thevigorousness of the vaporization, coagulation and surface diffusion ofsilicon and carbon in the heat treatment further depends on the crystalplanes with which the trench side walls coincide.

FIG. 5 is an electron micrograph observing the surface of a siliconcarbide semiconductor device according to a comparative example.

Comparative SiC semiconductor device 130, in which the first and secondconnection angles are set at 45 degrees of angle, is formed and treatedthermally. The manufacturing method for manufacturing SiC semiconductordevice 130 and the conditions, under which trenches are formed, are thesame as those for SiC semiconductor device 100 shown in FIG. 1. It isconfirmed that the trench is filled in connection portion 6 betweenstraight trench section 2 and connecting trench section 5. In otherwords, it is found that the trench is filled in the portion, in which acorner occurs on polygonal trench 11 shown in FIG. 4.

The observations described above indicate that the trench side walls areprevented from being deformed by the heat treatment conducted at 1500°C. or higher by shaping connecting trench section 5 with a combinationof a straight connecting trench extending at 30 degrees of angle withstraight trench section 2 and a straight connecting trench extending at90 degrees of angle with straight trench section 2 or a combination of astraight connecting trench extending at 60 degrees of angle withstraight trench section 2 and a straight connecting trench extending at90 degrees of angle with straight trench section 2. Still alternatively,connecting trench section 5 may be shaped with a combination of astraight connecting trench extending at 30 degrees of angle withstraight trench section 2, a straight connecting trench extending at 60degrees of angle with straight trench section 2 and a straightconnecting trench extending at 90 degrees of angle with straight trenchsection 2 for preventing the trench side walls from being deformed.

According to the first embodiment of the invention, trench 1 is formedin the SiC semiconductor device such that the end portions of thetrenches constituting straight trench section 2 are connected with firstconnecting trench 3 via connecting trenches extending at 30 degrees ofangle or at 60 degrees of angle with straight trench section 2. Byforming trench 1 as described above, the trench side walls are hardlydeformed and the trenches are prevented from being filled even by theheat treatment conducted at 1500° C. or higher. Since it is possible tomanufacture an SiC semiconductor device that includes no end portions ofthe trenches, it is possible to prevent the electric field fromlocalizing to the end portions of the trenches. Therefore, it ispossible to prevent the SiC semiconductor device from being deterioratedor broken down.

Second Embodiment

FIG. 6 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a second embodiment of theinvention. FIG. 7 is an electron micrograph that observes the surface ofthe silicon carbide semiconductor device shown in FIG. 6 before the heattreatment thereof.

The SiC semiconductor device according to the second embodiment may beobtained by adding fourth and fifth connecting trenches to connectingtrench section 5 of the SiC semiconductor device according to the firstembodiment.

In SiC semiconductor device 140 according to the second embodiment,second connecting trench 4 a connects the end portion of first straighttrench 2 a and the first end portion of fourth connecting trench 7 a.The second end portion of fourth connecting trench 7 a is connected tothe first end portion of first connecting trench 3. Third connectingtrench 4 b connects the end portion of second straight trench 2 b andthe first end portion of fifth connecting trench 7 b. The second endportion of fifth connecting trench 7 b is connected to the second endportion of first connecting trench 3. Fourth and fifth connectingtrenches 7 a and 7 b correspond to a fourth trench section.

The first and second connection angles are 30 degrees of angle. Fourthconnecting trench 7 a is extended such that the angle between theextension of first straight trench 2 a and fourth connecting trench 7 ameasured from the extension of first straight trench 2 a toward theinside of straight trench section 2 (hereinafter referred to as the“third connection angle”) is 60 degrees of angle. Fifth connectingtrench 7 b is extended such that the angle between the extension ofsecond straight trench 2 b and fifth connecting trench 7 b measured fromthe extension of second straight trench 2 b toward the inside ofstraight trench section 2 (hereinafter referred to as the “fourthconnection angle”) is 60 degrees of angle. In other words, connectingtrench section 5 in the SiC semiconductor device according to the secondembodiment is shaped with half the circumference of a dodecagon. Theother structure in SiC semiconductor device 140 according to the secondembodiment is the same as that of SiC semiconductor device 100 accordingto the first embodiment.

For the light exposure in manufacturing SiC semiconductor device 140, areticle, in which a trench pattern, 1.5 μm in width, is formed, isemployed. Since side etching occurs, the resulting trench width is 2 μm.The other details of manufacturing SiC semiconductor device 140 are thesame as those of manufacturing the SiC semiconductor device according tothe first embodiment.

As described above, the manufacturing method for manufacturing the SiCsemiconductor device according to the second embodiment exhibits theeffects same as those of the manufacturing method according to the firstembodiment.

Third Embodiment

FIG. 8 is an electron micrograph that observes the surface of a siliconcarbide semiconductor device according to a third embodiment of theinvention. FIG. 9 is an electron micrograph that observes the surface ofthe silicon carbide semiconductor device shown in FIG. 8 before the heattreatment thereof.

The manufacturing method for manufacturing SiC semiconductor device 150and the conditions under which trenches are formed are the same as thosefor SiC semiconductor device 100 according to the first embodiment. Itis effective to dispose a vertical and straight connecting trenchextending vertically to straight trench section 2 (hereinafter referredto as a “vertical straight connecting trench”) in substitution forconnecting trench section 5 in the SiC semiconductor device 100according to the first embodiment. The end portion formed inside trench1 by straight trench section 2 and the vertical straight connectingtrench is set to be polygonal. The widths of the end portions of thetrenches constituting straight trench section 2 are made to be widergradually from straight trench section 2 toward the vertical straightconnecting trench.

Straight trench section 2 is arranged, for example, in a stripe pattern.Vertical straight connecting trench 21 is disposed vertically tostraight trench section 2. For example, all the straight trench sections2 are connected to vertical straight connecting trench 21. The shapes ofthe end portions inside trench 1 formed by the side walls of straighttrench sections 2 and the inner side wall of vertical straightconnecting trench 21 continuous to the side walls of straight trenchsections 2 is polygonal. The end portions of the trenches constitutingstraight trench sections 2 become wider gradually from the side ofstraight trench sections 2 toward vertical straight connecting trench21. The other structure of the SiC semiconductor device according to thethird embodiment is the same as that of the SiC semiconductor deviceaccording to the first embodiment. Vertical straight connecting trench21 corresponds to a fifth trench section.

The shapes of the end portions inside trench 1 may be the same as theshape of the connecting trench section (cf. FIG. 1) according to thefirst embodiment or the second embodiment with no problem.

For the light exposure in manufacturing SiC semiconductor device 150, areticle, in which a trench pattern, 1.5 μm in width, is formed, isemployed. Since side etching occurs, the resulting trench width is 2 μm.The other details of manufacturing SiC semiconductor device 150 is thesame as those of manufacturing the SiC semiconductor device according tothe first embodiment.

The SiC semiconductor device shown in FIG. 8 facilitates preventingtrench 1 from being filled, even if the SiC semiconductor device weretreated thermally at 1700° C., since the end portion of trench 1 ispolygonal and since the trenches constituting straight trench section 2are widened gradually from the side of straight trench section 2 to theside of vertical straight connecting trench 21. It is found that thetrench in SiC semiconductor device 150 before the heat treatment thereofshown in FIG. 9 is narrowed locally in the connection portion betweenstraight trench section 2 and vertical straight connecting trench 21. Incontrast, it is confirmed that the trenches are not filled in SiCsemiconductor device 150 after the heat treatment thereof shown in FIG.8.

As described above, the manufacturing method for manufacturing the SiCsemiconductor device according to the third embodiment exhibits the sameeffects as those of the manufacturing method according to the firstembodiment.

Fourth Embodiment

FIGS. 10 through 13 are electron micrographs that observe the surface ofa silicon carbide semiconductor device according to a fourth embodimentof the invention. In trenches 1 shown in these drawings, the spacingbetween first and second straight trenches 2 a and 2 b (hereinafterreferred to as the “cell pitch”) constituting straight trench section 2is set at 3.8 μm in FIG. 10, 6.5 μm in FIG. 11, 10 μm in FIG. 12, and 15μm in FIG. 13. The polygonal shape of connecting trench section 5 may bechanged depending on the cell pitch in straight trench section 2 with noproblem.

As shown in FIG. 10, connecting trench section 5 in SiC semiconductordevice 160 is shaped with half the circumference of a hexagon. In otherwords, the structure of SiC semiconductor device 160 is the same as thestructure of SiC semiconductor device 100 according to the firstembodiment (cf. FIG. 1). It is preferable for the cell pitch in straighttrench section 2 to be from 1.7 μm to 2.5 μm. The reason for this willbe described later.

As shown in FIG. 11, connecting trench section 5 in SiC semiconductordevice 170 is shaped with half the circumference of a dodecagon. Inother words, the structure of SiC semiconductor device 170 is the sameas the structure of SiC semiconductor device 140 according to the secondembodiment (cf. FIG. 7). It is preferable for the cell pitch in straighttrench section 2 to be from 4 μm to 5.6 μm. The reason for this will bedescribed later.

As shown in FIG. 12, connecting trench section 5 in SiC semiconductordevice 180 is shaped with half the circumference of an octadecagon. Inother words, SiC semiconductor device 180 has more connecting trenchesthan connecting trench section 5 in SiC semiconductor device 170 (cf.FIG. 11). In detail, connecting trench section 5 in SiC semiconductordevice 180 is shaped with a polygon connecting 9 line segments to eachother.

In SiC semiconductor device 180, the angles between the connectingtrenches constituting connecting trench section 5 and straight trenchsection 2 are as follows. The angles between the connecting trenches,including the second connecting trench connected to first straighttrench 2 a and the first connecting trench vertical to first straighttrench 2 a, and first straight trench 2 a measured from the extension offirst straight trench 2 a toward the inside of straight trench section 2are 20 degrees of angle, 40 degrees of angle, 60 degrees of angle, and80 degrees of angle.

The angles between the connecting trenches, including the thirdconnecting trench connected to second straight trench 2 b and the firstconnecting trench vertical to second straight trench 2 b, and secondstraight trench 2 b measured from the extension of second straighttrench 2 b toward the inside of straight trench section 2 are 20 degreesof angle, 40 degrees of angle, 60 degrees of angle, and 80 degrees ofangle.

The other structure of SiC semiconductor device 180 is the same as thatof SiC semiconductor device 170. The connecting trenches extending at 20degrees of angle, 40 degrees of angle, 60 degrees of angle, and 80degrees of angle with the extension of straight trench section 2correspond to the sixth trench section, the seventh trench section, thefourth trench section and the eighth trench section, respectively. Inthis case, it is preferable for the cell pitch in straight trenchsection 2 to be from 8 μm to 12 μm. The reason for this will bedescribed later.

As shown in FIG. 13, connecting trench section 5 in SiC semiconductordevice 190 is shaped with half the circumference of a triacontagon. Inother words, SiC semiconductor device 190 has more connecting trenchesthan connecting trench section 5 in SiC semiconductor device 180 (cf.FIG. 12). In detail, connecting trench section 5 in SiC semiconductordevice 190 is shaped with a polygon connecting 16 line segments to eachother.

In SiC semiconductor device 190, the angles between the connectingtrenches constituting connecting trench section 5 and straight trenchsection 2 are as follows. The angles between the connecting trenches,including the second connecting trench connected to first straighttrench 2 a and the first connecting trench vertical to first straighttrench 2 a, and first straight trench 2 a measured from the extension offirst straight trench 2 a toward the inside of straight trench section 2are 11.25 degrees of angle, 22.5 degrees of angle, 33.75 degrees ofangle, 45 degrees of angle, 56.25 degrees of angle, 67.5 degrees ofangle, and 78.75 degrees of angle.

The angles between the connecting trenches, including the thirdconnecting trench connected to second straight trench 2 b and the firstconnecting trench vertical to second straight trench 2 b, and secondstraight trench 2 b measured from the extension of second straighttrench 2 b toward the inside of straight trench section 2 are 11.25degrees of angle, 22.5 degrees of angle, 33.75 degrees of angle, 45degrees of angle, 56.25 degrees of angle, 67.5 degrees of angle, and78.75 degrees of angle.

The other structure of SiC semiconductor device 190 is the same as thatof SiC semiconductor device 180. The connecting trenches extending at11.25 degrees of angle, 22.5 degrees of angle, 33.75 degrees of angle,45 degrees of angle, 56.25 degrees of angle, 67.5 degrees of angle, and78.75 degrees of angle with the extension of straight trench section 2correspond to the ninth through fifteenth trench sections, respectively.In this case, it is preferable for the cell pitch in straight trenchsection 2 to be 12 μm or wider. The reason for this will be describedlater.

For the light exposure in manufacturing the SiC semiconductor deviceaccording to the fourth embodiment, a reticle in which a trench pattern1.5 μm in width is formed is employed. The resulting trench width is setto be 1.5 μm by the trench etching. Any first bake is not conducted. Asecond bake is conducted at 125° C. for 15 minutes. The heat treatmentconducted on trench 1 after the formation thereof is a general heattreatment for improving the trench shape and for activating theimplanted atoms. The general heat treatment is conducted at 1700° C. for10 minutes. The other details of manufacturing the SiC semiconductordevice according to the fourth embodiment are the same as those ofmanufacturing the SiC semiconductor device according to the firstembodiment.

It is investigated how connecting trench section 5 is deformed by theheat treatment conducted at 1500° C. depending on the cell pitch instraight trench section 2. FIGS. 14 through 21 are electron micrographsobserving the surfaces of trenches formed to be circular before the heattreatment thereof. FIG. 22 is an electron micrograph observing thesurface of a part of the trench shown in FIG. 21 before the heattreatment thereof. FIGS. 23 through 30 are electron micrographsobserving the surfaces of the trenches shown in FIGS. 14 through 21after the heat treatment thereof. FIG. 31 is an electron micrographobserving the surface of a part of the trench shown in FIG. 30. FIG. 32is a table describing the curvatures of circular trenches and the shapesof the trenches before and after the heat treatment thereof. The tabledescribed in FIG. 32 summarizes the results shown in FIGS. 14 through31.

The distances corresponding to the diameters of the circular patternsdescribed in FIG. 32 assume the cell pitches in straight trench section2. The trench shapes described in FIG. 32 assume the shapes ofconnecting trench sections 5 after the trench etching and after the heattreatment.

By the manufacturing method according to the fourth embodiment,trenches, the sizes of which differ from each other as shown in FIGS. 14through 21 (hereinafter referred to as “first specimen 41 through eighthspecimen 48”), are formed on SiC substrates. The specimens are obtainedusing circular trench patterns, the diameters of which differ from eachother (hereinafter referred to simply as “circular patterns”) ascircular trenches, the curvatures thereof are different from each other.The diameters of the circular patterns for first through eighthspecimens 41 through 48 are 1.7 μm, 2.5 μm, 4 μm, 5.6 μm, 8 μm, 12 μm,26 μm, and 48 μm, respectively. These trenches have a cross sectionalshape as shown in FIG. 3. The heat treatment is conducted in an argongas, to which monosilane is added at the flow rate ratio of 0.4%, underthe pressure of 80 Torr and at 1700° C. for 10 minutes.

As the results shown in FIGS. 14 through 21 indicate, the shapes of thespecimens formed to be circular trenches are deformed as describedbelow. First through third specimens 41 through 43 are deformed to behexagonal. Fourth specimen 44 and fifth specimen 45 are deformed to bedodecagonal. Sixth specimen 46 is deformed to an octadecagon. Seventhspecimen 47 is deformed to a triacontagon. Eighth specimen 48 is shapedalmost with a circle as the expanded view of end portion 49 of a trenchopening shows (cf. FIG. 22).

It is found from these results that if a circular trench, the diameterof which is 26 μm or shorter, is formed, the trench shape after thetrench etching will be polygonal. The reason for this is that theetching speed depends on the crystal plane of the SiC semiconductordevice (due to the crystal plane dependence of the etching speed). Inother words, it is found that the trench side walls in the specimensshown in FIGS. 14 through 21 are hardly deformed by etching. It is foundthat the trench is shaped with a polygon having more corners afteretching, as the curvature of the circular trench is larger. In otherwords, it is found that the crystal planes hardly deformed by etchingare different depending on the curvature of the circular trench. It isfound that the crystal planes hardly deformed by etching increase as thecurvature of the circular trench is larger.

It is found from the results shown in FIGS. 23 through 30 that theshapes of the specimens formed to be circular trenches are not deformedby the heat treatment or deformed by the heat treatment to a polygon.First and second specimens 41 and 42 are not deformed but maintain thehexagonal shape. Third specimen 43 is deformed to be dodecagonal. Fourthspecimen 44 is not deformed but maintains the dodecagonal shape. Fifthspecimen 45 is deformed to an octadecagon. Sixth specimen 46 is notdeformed but maintains the octadecagonal shape. Seventh specimen 47 isnot deformed but maintains a triacontadigonal shape. Eighth specimen 48is not deformed but shaped almost with a circle as the expanded view ofend portion 49 of a trench opening shows (cf. FIG. 31).

It is found from the results shown in FIGS. 23 through 30 that some ofthe trenches formed to be circular are not deformed by the heattreatment but that the other trenches formed to be circular change theshapes thereof to a polygon having more corners. The reason why thetrench side wall is deformed to a polygonal shape by the heat treatmentis the same as the reason according to the first embodiment. In otherwords, it is found that the specimens shown in FIGS. 23 through 30include a trench side wall that coincides with the crystal plane hardlydeformed by a heat treatment. It is found that the trench is shaped by aheat treatment with a polygon having more corners, as the curvature of acircular trench is larger. In other words, it is found that the crystalplane that is hardly deformed by a heat treatment changes depending onthe curvature of the circular trench. And, it is found that the trenchside wall coincides with more crystal planes hardly deformed by a heattreatment, as the curvature of the circular trench is larger.

Therefore, it is found that it is possible to prevent a trench frombeing deformed by a heat treatment by shaping the trench with a polygonhaving crystal planes that are hardly deformed by the heat treatment. Itis found that it is preferable to assume a circular trench having adiameter of 4 μm after etching, for example, by a dodecagon that is theshape of the trench after the heat treatment thereof.

It is preferable to determine the polygonal shape of a trench after theetching thereof so that the shape of the trench after the heat treatmentthereof may be the same as the shape of the trench after the etchingthereof. By shaping the trench as described above, trench deformationscan be reduced through the steps for forming the trench in the SiCsemiconductor device. It is found that if a circular trench, thediameter of which is 1.7 μm, is approximated by a hexagonal trench, theshape thereof will not change before and after the heat treatmentthereof.

It is found from these results that connecting trench section 5 ishardly deformed by heat treatment by shaping connecting trench section 5with half the circumference of a hexagon, when the cell pitch instraight trench section 2 is from 1.7 μm to 2.5 μm. When the cell pitchin straight trench section 2 is from 4 μm to 5.6 μm, connecting trenchsection 5 shaped with half the circumference of a dodecagon is hardlydeformed by a heat treatment. When the cell pitch in straight trenchsection 2 is from 8 μm to 12 μm, connecting trench section 5 shaped withhalf the circumference of an octadecagon is hardly deformed by heattreatment. When the cell pitch in straight trench section 2 is 12 μm orlonger, connecting trench section 5 shaped with half the circumferenceof a triacontagon is hardly deformed by a heat treatment.

As described above, the manufacturing method for manufacturing an SiCsemiconductor device according to the fourth embodiment exhibits theeffects the same as those of the manufacturing method according to thefirst embodiment. Depending on the cell pitch, connecting trench section5 is shaped with a polygonal trench (that is half the circumference of ahexagon, half the circumference of a dodecagon, half the circumferenceof an octadecagon, or half the circumference of a triacontagon) havingside walls which coincide with the crystal planes hardly deformed by aheat treatment. If the SiC semiconductor device according to the fourthembodiment is treated thermally at 1500° C. or higher, the trench sidewalls will be hardly deformed and the trenches will be prevented frombeing filled at various cell pitches.

Now a semiconductor device including an n⁺-type source region and such aregion on a silicon carbide semiconductor device including trenchesshaped according to the invention will be described below. FIG. 33 is anelectron micrograph schematically showing a silicon carbidesemiconductor device according to the invention. FIG. 34 is a crosssectional view along the line segment X-X′ of FIG. 33. FIG. 35 is across sectional view along the line segment Y-Y′ of FIG. 33.

Herein, the description is made in connection with the SiC semiconductordevice according the second embodiment. As shown in FIG. 33, p⁺-typebase region 32 is formed on an SiC semiconductor substrate and n⁺-typesource region 33 is formed in the surface portion of p⁺-type base region32. The region of p⁺-type base region 32, in which n⁺-type source region33 is formed, works for an active region. The region, outside the activeregion, of p⁺-type base region 32, in which n⁺-type source region 33 isnot formed, works for an edge region. Straight trench section 2 isdisposed in the active region. The side walls of the trenchesconstituting straight trench section 2 coincide with the (1-100) planeand the (−1100) plane of 4H—SiC. Connecting trench section 5 is disposedin the edge region. Straight trench section 2 and connecting trenchsection 5 are connected to each other to form trench 1. The shape oftrench 1 is the same, for example, as that according to the secondembodiment. The p⁺-type base region 32 disposed in a channel region ispositioned inside trench 1.

In the cross sectional view shown in FIG. 34 along the line segment X-X′of FIG. 33, p⁺-type base region 32 is disposed on the SiC substrateworking for n⁻-type drift region 31. Connecting trench section 5 isformed through p⁺-type base region 32 down to n⁻-type drift region 31.The straight trench section not illustrated in FIG. 34 is formed down ton⁻-type drift region 31. In a part of the surface portion of p⁺-typebase region 32 in the channel region, n⁺-type source region 33 isdisposed.

In the cross sectional view shown in FIG. 35 along the line segment Y-Y′of FIG. 33, n⁺-type source region 33 is disposed in the surface portionof p⁺-type base region 32 in the channel region. The other structure isthe same as that in the cross section along the line segment X-X′ ofFIG. 33. A not-shown gate insulator film is formed in trench 1. A gateelectrode is formed on the gate insulator film. Thus, a vertical MOSFETis completed.

The trench side walls are made to coincide with the (1-100) plane andthe (−1100) plane of 4H—SiC according to the invention. Alternatively,the trench side walls may be made to coincide with the other crystalplanes that are hardly deformed by a heat treatment. For example, the(11-20) plane and the (−1-120) plane of 4H—SiC may be employed with noproblem. The semiconductor device obtained by using the silicon carbidesemiconductor device having the trench shape according to the inventionis exemplary. Therefore, changes and modifications will be obvious tothe persons skilled in the art without departing from the true spirit ofthe invention for meeting the use of the semiconductor device.

When the cell pitch in straight trench section 2 is 48 μm or longer,connecting trench section 5 may be shaped with a circle as shown in FIG.32. It is found from the results shown in FIG. 32 that connecting trenchsection 5 is shaped almost with a circle after the heat treatmentthereof.

As described above, the silicon carbide semiconductor device and themanufacturing method thereof according to the invention are useful forpower semiconductor devices having a trench structure and for themanufacture thereof.

Thus, a silicon carbide semiconductor device and the manufacturingmethod thereof have been described according to the present invention.Many modifications and variations may be made to the techniques andstructures described and illustrated herein without departing from thespirit and scope of the invention. Accordingly, it should be understoodthat the devices and methods described herein are illustrative only andare not limiting upon the scope of the invention.

What is claimed is:
 1. A silicon carbide semiconductor devicecomprising: a silicon carbide semiconductor substrate of a firstconductivity type; a trench, in a surface portion of the semiconductorsubstrate, end portions of the trenches being connected to each othersuch that a polygon is formed; a base region of a second conductivitytype disposed in adjacent to the trench; a source region of the firstconductivity type disposed in the base region; and a gate electrode inthe trench with a gate insulator film interposed therebetween.
 2. Thesilicon carbide semiconductor device according to claim 1, wherein eachinterior angle of the polygon formed by the trench is 150 degrees. 3.The silicon carbide semiconductor device according to claim 1, whereineach interior angle of the polygon formed by the trench is 160 degrees.4. The silicon carbide semiconductor device according to claim 1,wherein each interior angle of the polygon formed by the trench is168.75 degrees.
 5. A silicon carbide semiconductor device comprising: atrench, in a surface portion of a silicon carbide semiconductorsubstrate, end portions of the trenches being connected to each other; abase region disposed in adjacent to the trench and having a conductivitydifferent from the substrate; a source region disposed in the baseregion and having a conductivity different from the base region; and agate electrode disposed in the trench with a gate insulator filminterposed therebetween, wherein the outer sidewalls of the end portionsof the trenches form a straight and the inner sidewalls of the endportions of the trenches form a polygon.